Electron lens for electric discharge tubes



Oct. 28, 1941. E. BRUCHE 2,260,351

ELECTRON LENS FOR ELECTRIC DISCHARGE TUBES Filed June 28, 1938 2Sheets-Sheet 1 Inventor:

Ernst, BTUCEQ,

v H s Attorney.

Faglk E. BRUCHE Oct. 28, 1941.

ELECTRON LENS FOR ELECTRIC DISCHARGE TUBES 2 Sheets-Sheet 2 Filed June28, 1938 Inventor:

a .h VJ C e u/Wn P P lm .t n W m P E W Patented Oct. 28, 1941 ELECTRONLENS FOR/ELECTRIC DISCHARGE TUBES- Ernst Briiohe, Berlin-Reinicken'dorf,Germany, 'assignor to General Electric Company, a. corporation of NewYork Application June 28, 1938, Serial No. 216,368

" In Germany July'7,=1937 v 3 Claims.

The present invention relates to cathode ray tubes and more particularlytoelectron lenses which control the direction of travel and focus of theelectrons forming the beam.

It is known that on connecting a high-irequency alternating potential toa suitably constructed accelerating system in a cathode ray tube,charged particles may be given a velocity which is greater than thatcorresponding to-the amplitude of the alternating potential, The fact isthereby used that the particles in the system possess periods of motionwhich are of the same order asthe period of the alternating potential;Thus, a particle. for which exists at a given instant an acceleratingfield in the'one field part anda retarding field in the other fieldpart, may

actually continuously be accelerated, because it reaches the secondfield part at a later time than the first field part. In the meantime,the direction of the alternating: potential has been reversed, and theretarding field has'become an accelerating field. I

According to the invention, high-frequency alternating fields are usedfor the focusing efiects ofeleotronor ion lenses. The lenses accordingto" the invention difier from-the known arrangements using electricallenses operated by alternating potential in'that, with these, theperiodof the alternating potential is of the order of the time of motion ofthe electrons in the lens, while with theknown arrangements, wherealternating potentials of. network frequency have been used, the periodis only a fraction of the time of motion 'ofjthe v electrons from theposition of the object 7 to the position of the picture. I

Details. of the invention are explained by the aid of thedrawing inwhich Figs. 1 to 3 represent typical improved electron lenses togetherwith. a graphical explanation of their mode of operationand Fig. 4 is adiagrammatic representation of a cathoderay tube apparatus for usefullyapplying the invention. I Big. 1 shows an immersion lens, which accord:ing to Fig. 1a consists of two co-axial hollow cylinders In, which havedifferent potentials with respect to ground or. othe r fixed potential,The direction of the optical axis is indicated by 2-. Figs. v1b,,and 1cillustrate for two different con-- ditions of operation theinstantaneous variations of potential level which exist as one proceedsfromileft to right along the optical axis. Fig.,1b I represents-suchvariation for a retarding lens systemand shows that the negativepotential gent lens.

accelerating lens the negative potential level decreases'as shown inFig; 1c.

The symbol I placed above the axis a in 1b indicates that the lenseffect of. the-potential field to the left of the central plane ofthelens is such as to produce divergence'of the electron beam traversingthe-lens. The. symbol 0. placed above the axis a to the right of thelens center line, on the other hand, indicates a converging effect ofthe lens system. In Fig. 1c: the regions of'convergence and divergenceare reversed. It will be seen from the. illustration, that in bothlenses, i. e. the accelerating and the retarding lens, there are areaswith divergent; effect-and areas with convergent effect. It willfurthermore be seen that the divergent effect always appears in the areaof highelectron. velocity, which has the 1 result that the lens as awhole represents in both cares a convergent. lens. These considerationsmade with respect to: the immersion lens are true quite 'generallyifor;anytypeof the short immersion or single lens. Only by using aperturestop lenses'it is possible to make divergent lenses. But also whenusingaperture stop lenses' .,it..is not possibleto makechromaticallycorrect'ed lens systems.

If now an immersion lens is. operated with high-frequency alternatingpotential, the frequency and phaseof which is so selectedthat certainelectrons pass through the first-part of the immersion lens'during theone-half wave and through the second'part during theother half wave ofthe alternating potential, it is possible to obtain a pureconvergent'lens or a pure diver- This 'fact is explained by the :aid ofFigs. 1d and lesin the case of a pure divergent lens. Fig. 1d representsthe variation of the form of the potential field as a given electronprov gresses through the field,. the condition of the field beingindicated for nine equally spaced po-. sitions of the electron. Fig. 1erepresents the continuous variation of the applied'alternating potentialwith electron position. Consider first an electron which at a certaininstant .is at the p'ointsignified by the Roman numeralI ot Fig.

1d, and let the alternating potential assume the value at all points'asindicated by the dashxline associated with numeral linl 'ig. 1d.Dueto-its starting velocity the electron reaches the point II at atime-when the. alternating potential has at.-

tained a positive value such that the potential field is of thechara'cterdndicatedby the .doti dash level increases alongzthmopticalzaxis.. For-flan 55 curve-associated with rnumerale-zzin,.:..E'ig-.I..-1-d-.

Thus, in the area where the electron is that part of an immersion lenswhich corresponds to a divergent lens is formed. When the alternatingpotential obtains its maximum value, the electron is at the point III,so that now the most strongly curved potential curve 3 is formed. Duringthe next quarter phase of the alternating potential, the electron passesthrough the points IV and V, during which the field is decreasedaccording to the curves 4 and 5. When the electron passes now throughthe second part of the immersion lens, where there would be according toFig. 1b a convergent effect, the alternating potential has according toFig. 1e reversed its sign and has thus changed the second part of theimmersion lens into an accelerating part. While the electron moves fromV to IX, this accelerating field is according to the curves 5 to 9 builtup and then decreased. Also in the second part of the lens, the electronis then made subject to a divergent efiect.

By applying alternating potential, it is therefore possible to changethe immersion lens into a pure divergent lens. Correspondingly, if thephase of the alternating potential is shifted about 180, or the instant,at which the electron reaches the lens, is shifted about 180 of thispotential, the immersion lens may be changed into a convergent lens ofamplified effect. But the possibility of making a divergent lens is ofspecial importance, since a divergent lens similar to the opticaldivergent lens was hitherto not known in electron-optics, so that it wasalso not possible to construct an achromatic lens arrangement.

The idea of operation with alternating potential is, of course, notlimited to an immersion lens. Figs. 2 and 3 respectively represent asfurther cases a so-called single lens (Fig. 21:) consisting of threeelectrodes I2, 13 and I4 and an immersion objective lens (Fig. 3a")consisting of a cathode l5 and two cylinders 16 and Il. Figs. 2b" and3b" illustrate for these tWo cases the static potential fielddistribution at a given instant. Figs. 2d and 3d" respectively indicatethe variations in field form which occur for the two constructions underconsideration as a chosen electron proceeds through the lens space. Inconnection with the latter figures the assumed electron position isindicated by Roman numerals and the corresponding field form isindicated for each position by a curve which bears an Arabic numeralcorresponding to the Roman numeral associated with such position, Withthe field conditions indicated, a divergent lens action is obtained.Figs 2c and 36" show, for the two lens constructions to which theyrespectively pertain, the nature of the variation of potential magnitudewith electron position. These curves are based on the assumption of analternating potential which varies sinusoidally with time,

but the plotting of the curves is notagainst time described asdiscontinuous. In reality, of course, the form of the potential field(i. e. the electron lens field) varies continuously as the electronsprogress from point to point.

With alternating fields, the signification of the potential is not thesame as with static fields. It is more suitable in this case not toconsider the concept of potential and to work with the effect of force.In the static case, the force acting on an electron in the vicinity ofthe axis is in the case of a potential V=U(z) E=U"r, where the primesmean derivatives with respect to z and where 'r is the distance of theelectron from the axis. U may, for example, be the potential,illustrated by Fig. lb or 10, of the immersion lens. If now theimmersion lens is operated with alternating potential, the potential isof the form V=U(z) sin ft, and the force is therefore E=U"r sin ft, aswill be understood from the paper by Briiche and Recknagel in theZeitschrift fiir technische Physik, vol. 17, p. 126 n. (1936),particularly from the note 12 on page 132. Constant proportionalityfactors have here been neglected. While in the purely static case, thetype of lens effect, namely the sign of E, is solely determined by thesign of U", with the lens operated by alternating potential therehasstill to be added the sign of the sine-factor variable With time.Furthermore, the magnitude of the force, i. e. the magnitude of theconvergent or divergent effect existing at the time, is influenced bythe variable magnitude of this factor.

The lenses operated with alternating potential may, of course, be usedjust as well for positive particles as for electrons. In order to obtainthe same effect, only the potentials in the lens electrodes have to bereversed with respect to the potentials selected for the electrons.Also, magnetic lenses may be operated in the same manner. It is ofspecial advantage to use two coils connected in series. At a certainstrength of the current flowing in the same direction in both coils, itmay then be achieved that the picture rotation effected by the coils isso that the picture appears upright. But if the same coils are operatedwith alternating current, in such a manner that the electron issubjected to oppositely directed magnetic fields, the picture is notrotated, By optional operation of these coils with director alternatingcurrent, an upright or a reversed picture may thus be obtained asdesired.

Hitherto, the case has been considered that a sinusoidal potential or asinusoidal current serves to operate the lenses. It is, of course,possible to use potentials of different Wave shape, such as rectangularor triangular, obtained in a wellknown manner. The use of rectangularpotentials or currents has the advantage that the maximum possible lenseffect obtainable with a given field arrangement is achieved. If theimmersion lens discussed in Fig. 1 were operated by a rectangularpotential, the potential field would have the shape indicated at 3 inFig. 1d while the electrons pass through the first part of the lens, andin the second part of the lens would have the shape illustrated by l.The same thing is true on using magnetic lenses, since also in thiscase, the whole refractive power of the first lens, and then the wholerefractive power of the second lens, would become efiective.

It. is, of course, possible to combine the lenses operated byalternating potential according to the invention with. static electricor magnetic lenses. This is particularly of advantage for the tion 20and an enlarged bulbous portion 2|.

one end of the envelope there is provided an manufacture of achromatically corrected lens arrangement; since the convergent lens maybe produced by a known static lens, while the divergent lens necessaryfor the correction is pro-.

duced by an alternating potential lens. On using several lenses operatedby alternating potential v or alternating current, it is suitable to usea single voltage or current source. I In the leads to the single'lenses,devices may be provided which influence the phase of the potential oralter its shape. It is, for example, possible to use a potential sourcewhich produces a sinusoidal potential, and to operate only one of thelenses with this sinusoidal potential, while another lens, is operatedby aid of a rectangular potential.

certainranges of the electric phase, care has to be taken, that withimage devices using alternating potential lenses, the'charged particlesenter the lens only in these phases (i. e. during periods which includean instant of potential reversal). This might be achieved by the use ofan anode positioned near the cathode and having an acceleratingpotential taken from the ,same alternating current source as the lenspotentials. Also, devices may be inserted in the In Fig. 4 there isillustrated diagrammatically a complete cathode ray tube apparatus forusefully app ing the invention. This comprises an elongated envelopehaving a tubular shaft porelectron source comprising the combination ofa cathode 23, a first accelerating electrode 24 and a secondaccelerating electrode 25, A battery 2! serves to impress anaccelerating potential between the cathode 23 and the electrode 25. Atthe end of the envelope removefrom the electron source there may beprovided a fluorescent screen on the wall surface 28, as is conventionalin tubes of this character.

Between the electron source and the wall surface 28 there is provided anelectron lenssystem f the type illustrated in Fig. 3 hereof. Thiscomprises a first cylindrical electrode 30 which is arranged inpartially telesco'ped' relation with a second, generally cylindricalelectrode 3| consistpreviously explained, this expedient assures that Ia given electrode shall be subjected to the same. I directional effectthroughout theentire region of influence of the lens system. In orderthat electrons may enter the lens system only whenthe phase of thepotential applied thereto is favorable to the production of the desiredfocusing effect, use may be made of the accelerating'electrode 24. Inthis connection, the electrode may be connected to the source 33 whichsupplies potential to the lens system and may be energized thereby insuch fashion that it permits electron current to flow only at intervalswhich are calculated to bring the electrons into the lens system in theproper phase relationship. Known biasing means such as a battery 35 maybe used to assist in the performance of this function.

As a result of the means described in the foregoing, the electrons whichtraverse the lens systern may be focused in a desired fashion upon thewall surface 28. If desired, the apparatus may include magnetic orothermeans, illustrated dia- 1 grammatically at 31, for producing acyclical deflection of the thus focused beam.

which possesses utility mainly by virtue of its ability to control thelateral dimensions of the beam, said lens system comprising a pair ofmutually spaced apertured electrodes arranged'to be successivelytraversed by the beam, means for applying a potential between theelectrodes and for periodically reversing such potential to produce analternating lens field between said electrodes, and means for causingelectronsto pass through said lens field only during periods whicinclude an instant of potential reversal.

2. In a cathode ray tube, the combination which includes 'means forproducing a beam of electrons and an electron lens system whichpossesses utility mainly by virtue of its ability to control the lateraldimensions of the beam, said lens system comprising a pair of partiallytelescoped conducting cylinders arranged to be successively traversed bythe beam, means for impressing a potential between the cylinders and forreversing such potential with a periodicity which is of the order ofmagnitude of the time taken by the electrons in passing through the lenssystem to produce an alternating lens field between said cylinders, andmeans for causing electrons to pass through said lens field only duringperiods which include an instant of po tential reversal, wherebyelectrons are subjected to the same directional effect throughout theentire region of influence of the lens system.

3. In apparatus which includes means for producing an electron beam andin which it is neces sary for the proper functioning of the apparatus toexert a constant focusing effect on the various components of the beam,the combination of an electron lens system in the path of said beam forproducing the said required focusing of the beam by exerting transverserefractive forces on the various components of the beam, a source ofalternating potential for energizing the said lens system to provide analternating lens field, the operating frequency of the said source beingsufficiently high to assure thereversal of the potential in a periodwhich is of the order of magnitude of the time taken by the electrons inpassing through the lens field, and means for causing the electrons topass through said lens field only during periods which include aninstant of po- I tential reversal.

ERNST BRiicHE.

